The 24.3 translates to 24.30, to match the two decimals of 76.58, then you align them
24.30
76.58
Add the column on the very right: 0 + 8 = 8 ; then the one on the left 3 + 5 = 8; then the next one 4 + 6 = 10; and the one at the very left 2 + 7 = 9 but we add the 1 of the last 10, and leave the 0 on the place, so it's 10
24.30
76.58
---------
100.88
You bring the point down two numbers from right to left, like in the numbers we are adding